Although eurozone ministers agreed Tuesday to boost the value of the rescue fund, they acknowledged that they might call on the International Monetary Fund for help to do so after Italy's borrowing costs soared nearly to 8%, a euro lifetime high.

Those institutions and investors holding large quantities of Greek bonds may be in for bigger losses than an originally agreed-upon deal provided for, according to hints from European finance ministers who met Monday to discuss Europe’s ongoing debt crisis.

This was the first official acknowledgment that such an action might occur, and it was met with swift resistance from other euro zone officials. The effect on European banks and the European Central Bank (ECB), which all hold substantial amounts of Greek debt, would be huge. American banks also hold...